There are a number of advantages to storing a boat out of the water when it is not in use. Out-of-water storage prevents damage resulting from the boat bumping against adjacent docks, other crafts, or floating debris. It reduces the possibility of the boat breaking free from its moorage and either floating away or running aground. Out-of-water storage also lessens boat damage associated with long-term exposure to water and water-carried pollutants (e.g., corrosion, electrolysis, rusting, etc.), and the attachment of barnacles or other marine growth to the boat's hull. To store a craft out of water, many boat owners prefer to use a boat lift that simply raises the craft above the water's surface as opposed to the more involved procedure of dry docking the craft. Once a boat is lifted it can be maintained in a raised position for an extended period of time. A number of lift designs are currently known that provide this basic function.
U.S. Pat. No. 4,895,479 describes a lift for a watercraft that includes a base frame positioned underwater. The frame supports upwardly extending pivotable parallel arms. A lift platform is connect to the upper ends of the arms. The base frame, arms, and platform combine to form a parallelogram shape when viewed from the side. The lift is actuated by a hydraulic cylinder that is connected diagonally across the parallelogram. Extension of the cylinder rotates the arms about their lower end connection to the base frame. Thus, the rotation of the arms moves the lift platform (and hence any watercraft thereon) between raised and lowered positions. In its raised position, the pivotable arms are disposed under true vertical at an angle of approximately 70.degree. counterclockwise from horizontal. The pivotable arms are locked in this position by a cylinder locking mechanism that includes a pawl having a nose that is insertable into slots spaced along the cylinder's plunger. The nose is biased to enter a slot by a compression spring.
Another known boat lift is described in U.S. Pat. No. 5,184,914. The '914 lift has upwardly extending pivoting booms that are supported on a rectangular base submerged in water, similar to the '479 device. A watercraft support platform is connected to the upper ends of the pivoting booms. A double-acting hydraulic cylinder is connected diagonally between the rectangular base and the forward pivoting booms. The cylinder swings the pivoting booms upwardly until the booms are braced by boom stops located on the base. In the lift's fully extended position, the booms are braced against the boom stops at an angle of about 10.degree. overcenter (i.e., 10.degree. counterclockwise past vertical). Like the '479 lift, the '914 lift has a parallelogram configuration in side view.
Both the '479 and '914 devices suffer from a number of disadvantages. For example, the locking mechanism of the '479 device can be unreliable and hazardous, resulting in the unexpected and rapid lowering of a watercraft should the pawl release the plunger or otherwise fail. It appears that the '914 device attempts to overcome the deficiencies of the '479 cylinder locking mechanism by instead using the canted boom stops to brace the pivoting booms in an overcenter position.
A second disadvantage is that the diagonal placement of the actuating components in the '479 and '914 devices limit the minimum lowered position height that may be attained. It is desirable for a lift to be capable of being moved to a low height, to enable a user to drive a boat onto the lift in shallower waters. This does not appear to be possible with the '479 and '914 lifts, since both devices are configured with their actuators oriented between the middle of a rear transverse beam and the middle of a cross member at the forward boom arms. For the cylinders to effectively raise the arms, the force exerted by the cylinders must include a significant vertical force component at all times. This is especially true when the lift is in its lowered position, otherwise the cylinder will not be able to raise the arms, but will instead jam against the connection to the forward booms when the cylinder attempts to extend. It is this required vertical component that prohibits the lift from lowering further. The '914 lift has an additional disadvantage in that its canted boom stops further limits the minimum lowered height attainable.
A third disadvantage is that both devices appear to be using low pressure hydraulic water cylinders. These large cylinders are typically not sealed well enough to maintain the lift in any particular position. The '479 lift compensates for this insufficiency by including a cylinder locking mechanism. The '914 lift compensates by positioning the support platform overcenter, braced against the canted boom stops.
A fourth disadvantage of the '479 and '914 devices is the mid-lateral placement of their hydraulic cylinders. At this location, the cylinders can interfere with the keels and skegs of certain watercraft, thereby limiting and restricting the lift's utility to shallow keeled and shallow skegged crafts.
Accordingly, there exists a need for a watercraft lift that can safely, securely, and reliably support a watercraft in a lowered position, a raised position, and all points in between. The ideal watercraft lift should be operable in relatively shallow water, and should accormodate watercraft having deeper keels and skegs. The lift's minimum height should not be limited by the actuating components, so that the lift can be used in shallower waters. The present invention is directed to fulfilling this need and to providing further related advantages.